Typically, communication devices, for example, wireless cellular phones, include a communication sub-system and an audio sub-system. The communication sub-system includes a transceiver operably coupled to an oscillator while the audio sub-system includes an audio amplifier, such as a switching amplifier, and a modulation unit. The modulation unit, conventionally, works on a duty cycle modulation scheme such as pulse width modulation (PWM) to provide one or more PWM waveforms of an input signal, for example, a speech signal or a music signal. The switching amplifier receives the PWM waveforms from the modulation unit and amplifies them. Further, the switching amplifier provides the amplified PWM waveforms to drive a load such as a speaker.
Generally, the communication sub-system is isolated from an audio sub-system by assembling both the sub-systems on different chips. However, aspects such as cost, size, performance, and power are increasingly characterizing communication devices, and cellular phones in particular. These aspects are feasible only if a high degree of integration is possible. Therefore, instead of having a three-chip or two-chip solution with a few external components, efforts are being made to integrate the communication sub-system and the audio sub-system on a single chip.
Integrating the transceiver and the oscillator onto a single chip along with the switching amplifier is particularly difficult because of a phenomenon called injection locking or frequency pulling, introduced in the oscillator by the switching amplifier, such as a class-D amplifier. Typically, the class-D amplifier is used in audio applications because of its high power efficiency and low power dissipation. During operation, the class-D amplifier drives the modulated waveform to saturation and cut-off modes at a high switching speed, generating a rectangular waveform with fast moving transition edges as an output. Such high switching leads to creation of time varying interference loops. These interference loops vary the operating frequency of the oscillator due to mutual inductive coupling. This phenomenon is referred to as frequency pulling. Frequency pulling may also be observed in two-chip and three chip solutions if the transceiver is in physical proximity to the oscillator.
Due to frequency pulling in the oscillator, the oscillator introduces adjacent spurs in the transceiver, which can de-modulate adjacent interfering channels along with the desired channel. The adjacent spurs can also transmit spurious signals in adjacent frequency bands in addition to the desired frequency band, thus, violating the various standards of data transmission.